This invention relates to the field of optical to electrical signal transducer devices and their fabrication in the form of an improved metal semiconductor metal (MSM) device.
High data rate optical communications systems require high-speed optical receivers for information transmittal. These receivers must have an integral high-speed optical detector to convert the optical signal to an electrical signal. Semiconductor diodes are currently preferred for use as these detectors. The semiconductor Indium Gallium Arsenide (InGaAs) grown lattice matched or near lattice matched to Indium Phosphide (InP) is a desirable material for such diodes due to its internal electron speed and its efficient absorption of radiation in the 1.3-1.6 micrometer wavelengths favorable to optical fiber communications. The presence of metal electrodes on the front surface of currently available embodiments of these devices, however, reduces their optical collection efficiency through shadowing. Backside illumination of such detectors is therefore to be preferred.
Semiconductor PIN photodiodes are currently the most widely used detector for optical communications. These diodes must comprise regions of both n-type (electron rich) and p-type (hole rich) semiconductor material. Although this material requirement is compatible with bipolar transistor technology, it is highly inconsistent or non-compatible with Field Effect Transistor (FET) technology. To be integrable with the now preferred FET circuits for amplifying, switching and other electrical signal processing functions, the detector must be unipolar in nature.
Photoconductors are unipolar detectors but suffer from very slow response times and often have high dark current characteristics--a feature which reduces available signal-to-noise ratio. Metal Semiconductor Metal (MSM) detectors are back-to-back Schottky diodes typically formed as interdigitated metal fingers (gates). These devices are fully compatible with FET processing yet do possess disadvantages: the metal fingers shadow illumination from reaching the absorbing InGaAs region within the device. These devices also have the disadvantage of providing no electrical signal gain. In many uses, however, high optical signal levels and electrical amplification can be used to overcome this disadvantage.
Even though the photo FET device, especially the versions of the photo FET disclosed in the above-identified copending patent documents, overcomes this zero electrical gain characteristic of a photodiode, there is yet a large need for an improved photodiode device in the electronic art. The structure and theoretical simplicity of such photodiodes, especially when configured according to the present invention, and their relatively small size assures continued application of such diodes in electronic systems for an extended time.
MSM devices in frontside illuminated configuration have been known in the semiconductor device art for several years. A device of this type which further includes transparent electrodes in order to prevent electrode shading effects is described by R. N. Simons in the IEEE Transactions on Microwave Theory Technology, Vol. MTT-35, p. 144, 1987. Devices of this type have incurred process-related difficulties, however, and also tend to show large leakage currents.
The prior art has therefore not provided an MSM photodetector device which provides the increased optical energy response, ease of fabrication and other desirable characteristics that are available from the replaced substrate, flipped over, and backside surface illuminated MSM devices of the present invention.